This invention concerns a device and method for interrupting, i.e., breaking an electrical supply circuit to terminate current flow therethrough in response to the detection by the device of a fault located within the wiring connecting the load device and a power source which in one example is manifested as an atypical voltage at the load device end of the circuit when compared to the source voltage.
Electrical circuits of buildings of all types, particularly residences, which typically employ insulated wiring or cable are susceptible to damage, deteriorated connections, or the overloading of extension cords which generate sufficient electrical resistance to ignite the electrical insulation or adjacent building structure such as wood studs, rafters, wall board or the like. Such damage can occur, for example, during construction or remodeling of a residence whereby a nail, screw or the like may be driven into the wiring and diminish its current capacity. Such damage can go unnoticed during the operation of a lamp, power tool, electrical heater or the like even though the increased resistance in the damaged area may be generating heat and sparks inside a wall or the like and may start a hidden fire. More often, however, it is thermal expansion and contraction, presence of moisture, or dissimilar metal corrosion which causes connections in circuits serving higher amperage devices and appliances to develop resistance and heat.
Heretofore, considerable effort has been directed toward protecting the individual from electrical shock resulting from short circuiting or the like of the equipment being used, or toward protecting the equipment itself, i.e., protected circuit, by means of circuit breaking devices which measure current at a single point near the electrical breaker and interrupt power to the equipment when an abnormal electrical condition occurs. However, fires often begin with perfectly functioning current limiting circuit breakers in place. The detection of certain fault conditions, such as a moderate to high resistance fault in the wiring in the walls of a building, may elude detection by the traditional means and result in serious fire risks.
Various devices and methods which have been devised for protecting against ground fault and the like are shown, for example, in U.S. Pat. Nos. 3,780,348; 4,012,668; 3,648,111; 4,979,070; 3,386,004; 4,079,344; 4,816,957; 4,216,516; and 4,574,324. None of this prior art, however, is directed to disconnecting a load device from the electrical circuit of a residence or other building upon detecting a damaged line or oxidized connection within the circuit which could lead to resistive heating and incendiary conditions.
There is a need for a device that will detect fire-causing electrical conditions within the internal wiring of a circuit connected between a load device and the power supply of a residence or other such building and in response to such detection, will disconnect the load device from the circuit. Additionally, there is a need for a device to disconnect a load device from a power supply upon the detection of a fault condition that can distinguish between an actual fault within said supply circuit and a transitory, short-lived voltage or current aberrations existing within the power source of the supply circuit and can delay shut-down of the supply circuit for a predetermined power source connection period.
Further, there is a need for a device that can determine whether a fault condition exists within the wiring to prevent some of the fires which occur with prior art designs.